1. Field of the Invention
This invention relates to an improved engine control system for an automotive engine and, in particular, an automotive engine equipped with an automatic transmission. The engine control system of this invention is designed to interrupt fuel supply to the engine while the engine operates under normal engine load conditions in which a throttle valve opens so as to prevent the speed of the engine from being raised beyond an "over-revolution" restrictive speed.
2. Description of Related Art
In order to prevent an automotive engine from operating in an "over-revolution" condition, i.e., at rotational speeds beyond a specific rotational speed for which the engine is rated, it is typical to establish an upper critical speed for the automotive engine so as to prohibit an undesirable rise in rotational speed beyond the upper critical speed. Such an upper critical speed is referred to as an over-revolution restrictive speed in this specification. Prohibiting such an undesirable speed rise is performed by interrupting or cutting fuel supply to the engine, by controlling fuel ignition, or by controlling the rate at which intake air is admitted into the engine. An engine speed control system having an over-revolution prevention feature is known from, for instance, Japanese Unexamined Utility Model No. 59-194,550. On the other hand, because engine lubrication qualitatively deteriorates with a decrease in engine temperature, it has been proposed to lower the over-revolution restrictive speed, depending on a decrease in engine temperature, so as to realize reliable engine lubrication over allowable engine speeds.
Automotive engines require a certain speed control so that an over-revolution restrictive speed becomes lower for low engine temperatures. Such a speed control is hereafter referred to as an over-revolution prevention control, and is provided to protect the engine against deterioration of engine lubrication at low engine temperatures. Executing the over-revolution prevention control at low engine temperatures, however, may cause an automatic transmission to fail to up-shift in response to a requirement for quick acceleration. Moreover, an over-revolution prevention control system for an engine equipped with an automatic transmission may also have the disadvantage of letting the engine operate too sluggishly in speed for a long period of time to cause an up-shift, even if quick acceleration is needed. This is because the engine is normally forced to maintain a higher speed while operating at lower temperatures. This causes the shift-up vehicle speed to become higher, thereby heating up the engine. However, since the over-revolution restrictive speed established for lower engine temperatures remains low, although the shift-up vehicle speed is changed so that it becomes higher, no up-shift of the automatic transmission takes place, even though the engine operates at full throttle.